This invention relates to a composite sheet comprising a sheet of polymerial material having at least one of piezoelectric and pyroelectric types of stimulated current producing properties sandwiched between blocks of conductive (usually metallic) material. The composite sheets, preferably manufactured in large sizes, are adapted to detect stimulation, and are particularly suited for covering large areas such as a wall or walls enclosing a room, such as a "safe room", for securing it against certain intrusion. The sheet act as a transducer to intercept mechanical or thermal stimulation and produce an electrical voltage.
By way of background, the properties of crystalline substances having either piezoelectric or pyroelectric characteristics are well known. Piezoelectric materials have a low efficiency in mechanical to electrical conversion in response to applied forces. Pyroelectric material has a heat to electrical conversion. It is known how to produce these material in sheet form to have the desired properties. Some sheets will exhibit both piezoelectric and pyroelectric characteristics.
Polyvinylidene fluoride (PVDF) is a polymer with excellent piezoelectric and pyroelectric properties. In sheet form, it may be manufactured into a transducer by coating opposite sides with a thin flexible metallic surfacing and polarizing. Wire leads are attached to each of the metallic surfaces from which an output voltage is obtained whenever the sheet form transducer is stimulated. The larger the sheet, the larger the capacitance between the two metallic surfaces. This higher capacitance, unfortunately, decreases the output of the transducer at high frequencies. The reason for this will be explained later in the specification. This characteristic presents a substantial problem which is minimized in the present invention. Also, if a sheet is damaged in such a way as to short-circuit the two large opposing metallic surfaces, the transducer will produce no electrical output, and is, therefore, rendered totally ineffective.
Such a condition for a large sheet of the size indicated herein for wallpapering a room, or other area, is not practically acceptable The sborting condition is also a substantial shortcoming for the arrangements disclosed in U.S. Pat. No. 4,283,461 wherein a puncture or tear of the coating or film would cause a short circuit which would render the device inoperative. Accordingly, the invention disclosed herein represents a substantial improvement to overcome shorting and improve frequency response. It limits the effect of electrical shorting to the immediate area of the film covered by the immediate overlying blocks and also limits the loss of frequency response as the sheet grows in size.